1. Field of the Invention
The present invention relates to a transformer coupled amplifier circuit that eliminates transformer induced non-linearities by way of a unique dual feedback path without sacrificing any of the inherent attributes of general transformer coupled amplifiers.
2. Description of the Prior Art
Traditionally, in audio circuitry any situation calling for long wire runs or a very good source/load isolation automatically dictated the use of a transformer at the send and receive end of the wire run. While the transformer effectively isolated the load from the drive source, (up to the physical limitation of the particular transformer involved), it introduced severe signal degradation each time a transformer in/out pair was employed. In very early circuit applications, this signal degradation tended to be less than that of the driving/receiving circuitry. With the coming of solid-state circuits utilizing large amounts of negative feedback, the signal degradation occurred exclusively inside the transformer. Typical prior transformer coupled amplifier circuits of this type are shown in FIGS. 3, 4, and 5 between drive circuits and driven circuits and including transformers T, amplifiers A, and a number of resistors R. In the circuit shown in FIG. 3, the signal transfer function of the transformer T is not linear and this transfer function is well documented. Some amount of optimization could, however, allow the transformer T to perform reasonably well over some portion of the audio spectrum (20 Hertz to 20,000 Hertz). The major problems occur at frequencies below 250 Hertz. High non-linearity and phase shift become nearly intolerable for high quality audio applications. These problems are due primarily to decreasing impedance of the primary winding (since it is inductive). The only way to correct for this problem was to increase the number of turns of wire on the windings or increasing the amount of ferrous material in the core. Either solution results in a transformer far too large for use. What this meant to audio circuit designers is that for the ultimate in performance, transformers were to be avoided. Even with the most perfectly optimized transformers, distortion and phase shift were always present at audio frequency extremes and/or high signal levels. And the designers couldn't have a circuit with all parameters optimized, while keeping that total transformer isolation.
It was discovered that if one could "pick-off" part of the magnetic (highly nonlinear) signal within the transformer and apply it along with normal negative feedback signal, a percentage of the non-linearities could be cancelled. In some cases a totally separate winding used only for negative feedback was employed. This resulted in circuits similar to that shown in FIG. 4. In this circuit, the resistor R.sub.1 applies a feedback signal to the amplifier A and causes a reduction in non-linearities in driven circuit. The ideal situation where the resistor R.sub.F equals infinite ohms, would totally derive the feedback signal from the transformer. In real circuits R.sub.F must be some finite value in order to provide a direct current negative feedback path for the amplifier. For the circuit shown in FIG. 4, the direct current gain equals the value of the resistor R.sub.F divided by the value of the resistor R.sub.IN. Normal amplifiers having input offsets of several millivolts could develop large values of direct current voltage at point X in the circuit. Since, for direct current, the transformer windings present minimal resistance depending on the value of the resistor R.sub.G, output overload can occur very rapidly for high values of the resistor R.sub.F and hence, large amount of direct current. As we lower the value of the resistor R.sub.F we diminish the effect of the resistor R.sub.1 and subsequently swamp-out all the benefit derived from employing the transformer signal feedback path. Another factor in such a circuit is stability. As the resistors R.sub.1 and R.sub.F approach each other, in value, it becomes increasingly difficult to create a stable circuit offering any advantages.
In the circuit shown in FIG. 5, a differential amplifier A actually "watches" the transformer output and derives a negative feedback signal that is applied through the resistor R.sub.1. Immediately, the total isolation offered by the transformer T is lost due to the attachment of the amplifier A.sub.2. In addition, the swamping-out effect of the improvement derived from the amplifier A.sub.2 path, by the direct current feedback path of the resistor R.sub.2, causes only a small amount of improvement.
None of the prior art transformer coupled amplifier circuits, taken as a whole, disclose or suggest the present invention.
It will be understood that in the drawings of the circuits in FIGS. 3, 4, and 5 that R.sub.G models the resistive component of the input windings of the transformer.